The present invention relates to an optical modulation element which modulates light incident thereon from the external.
The fiber optic communication technology has developed through utilization of the ultra-low loss property of optical fibers and the ultra-wide band property intrinsic to light, and the fiber optic communication is now under study for longer-distance, large-capacity transmission in many countries. Now that the transmission loss of the optical fiber has reached a theoretical limit, studies for higher-speed, larger-capacity transmission are particularly important.
At present, high-speed ON-OFF control of an optical signal is generally effected by a method of directly modulating a semiconductor laser. With the direct modulation system, however, since a current of the semiconductor laser serving as an oscillation element is caused to vary at high speed, its oscillation wavelength changes significantly with time, resulting in the spectral width of oscillation becoming abnormally wider than the spectral width of the modulation band. In consequence, in a long-distance or high-speed transmission optical pulses to be received are distorted largely through the influence of wavelength dispersion by the optical fiber, and hence no excellent transmission characteristic can be obtained. To avoid this, there has recently been studied a method of modulating the semiconductor laser at high speed by an external optical modulation element while holding the laser output constant.
As the optical modulation element, there have been proposed an optical modulation element using a ferrodielectric substance such as LiNbO.sub.3 and an optical modulation element which can be integrated monolithically with a DFB laser or similar single-wavelength semiconductor laser. Among these optical modulation elements, the latter electroabsorption type optical modulation element in which an electric field is applied to a modulating waveguide layer to perform an intensity modulation of the output light by an electroabsorption effect is regarded as the most promising element.
However, the conventional electroabsorption type optical modulation element is capable of low-voltage modulation, high-speed operation and narrow-band oscillation when the intensity of incident light is low, but it is defective in that these characteristics significantly deteriorate when the light intensity is raised to a practical value of several mW.